I am 5'6" and use a 55cm ball. It seems fine for me.
Here is a chart: STABILITY BALL RECOMMENDATIONS
Ball Diameter Person's Height
30 cm - 11.8" - Under 4'6"
45 cm - 17.7" - 4'6" to 5'0"
55 cm - 21.6” - 5’1” to 5’7”
65 cm - 25.5” - 5’8” to 6’1”
75 cm - 29.5” - 6’2” to 6’7”
85 cm - 33.5” - 6’7” and up
ALL BALLS SHIPPED DEFLATED.
SEE OUR SELECTION OF AIR PUMPS.